This invention relates to maximizing the brightness of wood pulps bleached or brightened with bleaching agents, especially anionic bleaching agents so that brighter pulps may be obtained. This invention more particularly relates to maximizing the brightening of high-yield wood pulps used for newsprint and magazines.
Wood pulps can be classified as chemical pulps or high-yield pulps. Chemical pulps are prepared by chemically dissolving the non-cellulosic portion of the wood, such as lignins and hemicellulose. The process results in a purified wood pulp composed of 80-100 percent cellulose. High-yield wood pulps leave the bulk of the non-cellulosic components of the wood with the fiber because separation of the wood material into fibers occurs mainly through mechanical attrition of the wood, and contains up to about 60 percent cellulose.
High yield wood pulps are comprised of groundwood (GW) and refiner mechanical pulps (RMP). Stone groundwood pulp is produced by pressing logs against an abrasive rotating grindstone. Pressurized groundwood (PGW) is made by a modification to the stone grinding process; the logs are ground in a pressurized atmosphere to prevent the flashing of steam and to increase the temperature used in the grinding process.
Refiner mechanical pulps are produced by grinding wood chips between rotating grooved discs. Wood chips are introduced into the open eye of the refiner. As wood mass moves from the center of the refiner to the periphery, the wood is broken down into progressively smaller particles and finally into fibers.
Thermomechanical pulping (TMP) is a modification of the RMP process which involves steaming the wood chips for a short period of time prior to refining. Frequently both the heating and refining stages are done under pressure, but some pulping systems refine under atmospheric pressure (TRMP). Chemical treatments prior to refining or during the heating stage are further modifications of these pulping systems.
High-yield wood pulps are desirable for their low manufacturing costs and generally satisfactory physical properties. Their preparation involves very little loss of the original wood, and production processes typically are less expensive than chemical pulps. High-yield wood pulps are commonly used in newsprint, printing papers, molded products, corrugated paper, boxboards, and other applications.
While retaining the bulk of the wood material is economically favored, high-yield wood pulps tend to be dark and require bleaching in order to be used in certain applications. Various reducing and oxidizing agents may be used to brighten such high-yield wood pulps.
Oxidative bleaching with hydrogen peroxide generally produces higher brightness gains than those achieved with reductive bleaching processes. Brightness gains of 10-18 percentage points are attainable with hydrogen peroxide. The problem is that oxidative bleaching processes are more expensive than some of the other methods.
Reductive bleaching with compounds such as sodium dithionite (also called sodium hydrosulfite) or formamidine sulfinic acid (FAS) optionally in the presence of additives such as sodium tripolyphosphate, trisodiumnitrilotriacetate, and tetrasodium ethylenediaminetetraacetate also may be used. While reductive bleaching is usually cheaper, the brightness gains using these methods may be lower.
As will be appreciated by those skilled in the art, brightness is one of the most important product specifications in making commercially acceptable paper. When large brightness gains are not required, brightening with sodium dithionite bleaching is preferred over hydrogen peroxide because of cost. Manufacturers are under increasing pressure, however, to improve the brightness of their papers. Decreasing wood quality and environmental constraints have made these higher brightness targets more difficult to achieve. In balancing cost and need for increased brightness, many paper manufacturers are under increasing pressure to use the more expensive oxidative bleaching processes in order to meet the demand for acceptable products.
It is, therefore, an object of this invention to provide a process for obtaining enhanced bleaching of wood pulps in which reductive bleaches such as sodium hydrosulfite are used. It is a further object of the invention to extend the normal range of brightness gains for oxidative bleaches such as hydrogen peroxide.